Inband tone signaling using combinations of discrete frequencies has long been used in the telephone system. The primary advantage of inband tone signaling is that it shares the same spectrum that normally carries customer speech to transmit signal and control information. Sharing the voiceband is essential in situations where bandwidth is limited or dedicated control channels are either too costly or degrade the quality of voice transmission. One of the most common inband tone signaling systems in use in the telephone network today is touch tone dialing.
All inband tone signaling systems are premised on the belief that tone signals can be reliably detected. In touch tone dialing, for instance, reliable and accurate detection of 16 dual tone combinations is necessary to prevent improper routing of call attempts. Tone signal detector reliability translates into three basic performance criteria. The first and obvious criteria is the detector's ability to receive tone signals over the entire range of expected signal characteristics. These include the physical layer parameters such as tone signal level, frequency and duration. The two other performance criteria are somewhat unique to tone signal detectors and are meant to address the problems of receiver talkoff and talkdown.
One of the most common problems with inband tone signal detection is receiver talkoff. Talkoff occurs whenever a tone signal detector erroneously accepts a signal imitation produced by either speech or music as a valid alerting signal. Studies, experimentation and experience have all decisively confirmed that human speech can imitate some of the spectral and temporal properties of tone signals. The combination of consonants, vowels and syllables that frequently occur in an ordinary telephone conversation can cause a tone signal detector to talkoff. One of the challenges in designing a tone signal detector is making the detector non-responsive to these signal imitations.
Another problem associated with inband tone signal detection is receiver talkdown. Talkdown occurs whenever a tone signal detector fails to recognize a valid tone signal because it was masked by extraneous energy present on the line. In some situations, tone signals may have to compete with speech, music and other background noise. The presence of these complex signals distorts valid tone signals and impairs their detection.
Talkoff and talkdown are two critical measures of performance for a tone signal detector. They respectively describe the detector's ability to resist signal imitations and to recognize valid tone signals obscured by speech, music or noise. In the telephone network, emphasis has been traditionally placed on receiver talkoff performance because talkdown problems were circumvented by minimizing all interference on the line during tone signal pulsing. In touch tone dialing, for example, the transmitter of an ordinary telephone is muted when a dialing key is pressed. This prevents any customer speech or background noise from distorting the dual tone signal and allows central office receivers to be optimized to prevent talkoff during the intervals between key presses. Good talkoff performance was necessary for touch tone receivers but good talkdown performance was not necessary.
In recent years, new telephone services and advanced screen based telephony platforms such as the Analog Display Services Interface (ADSI) have been developed that require reliable tone signaling between the Stored Program Control Switching System (SPCS) and the Customer's Premises Equipment (CPE). These services and platforms, encouraged by many technological advances, are transforming the conventional telephone set into a sophisticated integrated terminal with a liquid crystal display and microprocessor, if not digital signal processor, with intelligence. One such service that depends upon reliable tone signal detection is Calling Identification Delivery on Call Waiting (CIDCW).
CIDCW is a service that complements the popular on-hook Calling Identification Delivery (CID) service by providing the calling line identification in the off-hook case for calls that are waited by the call waiting feature. For example, two parties, a near end party and a far end party, have a stable connection established between them and the near end party is a subscriber to the CIDCW service. When a third party trying to call the near end party finishes dialing the near end party's number, audible ringing is returned to the third party. A telephone switch recognizes that a call is destined for the near end party and begins to execute the CIDCW service routine. The switch line splits the connection, essentially muting the far end party. A tone generator is attached to the near end party's line at the end office and the normal call waiting signal, a burst of about 300 ms of a 440 Hz tone, is sent to alert the near end party that a call is waiting. Appended to the call waiting signal is a short burst of a special machine detectable alerting signal intended to prompt the near end party's CPE. Upon detection of this special tone signal, also known a CPE Alerting Signal (CAS), the near end party's CPE mutes the handset, sends an acknowledgment tone signal (ACK) back to the SPCS, places an FSK data receiver on the line and awaits the calling line identification information. The acknowledgment tone signal completes the tone signal handshake with the SPCS and signals the SPCS to begin data transmission. Shortly after data transmission is complete, the SPCS reconnects the near end party and far end party. At about the same time, the near end party's CPE unmutes the handset, decodes and displays the calling line information. The near end party can then make a decision as to whether or not he or she wants to take the incoming call.
The key to the successful operation of the CIDCW service rests in the reliable detection of the tone signals exchanged between the SPCS and CPE. Because of the nature of the signaling environment, the weakest link in the tone signal handshake is the detection of the CAS. For applications like CIDCW, good CAS detector talkoff and talkdown performance is critical.
CAS detector talkoff is a prime consideration in CIDCW. Because CIDCW is a service that can asynchronously interrupt a stable two way call at any time, the CAS detector must remain active awaiting a CAS signal for the entire duration of the call. This places the CAS detector at the risk of talkoff because it is constantly exposed to speech from both the near end and far end parties. In general, the longer a tone signal detector is exposed to speech, the greater the chance of talkoff. In CIDCW, CAS detectors are placed under an extreme talkoff condition.
Another factor that makes CIDCW an extremely difficult application for CAS detectors is the level of the pre-emphasized near end party speech. Speech level is one factor that negatively affects all tone signal detectors regardless of the frequency or temporal pattern of the tone signal. For a typical detector, the number of talkoffs that occur within a given period of time increases exponentially with increasing speech levels. Low speech levels are still quite capable of producing talkoffs, however, they are much less common. For this reason, system designers try to limit the level of speech incident upon the tone signal detector. In CIDCW, the CAS detector is "blasted" with speech from the near end party, speech that is heavily pre-emphasized by the near end party's telephone transmitter. The telephone transmitter provides increased gain at frequencies in the upper portion of the voiceband to counteract the effect of the typical loop and aids in making the speech more intelligible. However, it also raises the level of potential talkoff signals in the upper voiceband.
The impact of a CAS detector talkoff in CIDCW is a degradation of the quality of normal telephone service. If the CAS detector is talked off and erroneously accepts a signal imitation produced by speech, the near end party's CPE will interrupt the conversation by muting the handset and sending back an acknowledgment tone signal. Since the SPCS did not originate the CAS, the far end party will unintentionally receive the acknowledgment tone signal at a very undesirable listening level. Furthermore, the connection between the near end party and far end party remains interrupted until the CPE times out waiting for data. This interval is on the order of one half of a second. To prevent these unnecessary and disrupting interruptions, CAS detector talkoff must be minimized.
Unlike previous tone signal detectors used in the telephone network, the CAS detector for CIDCW also needs to exhibit good talkdown performance. Although the far end party is muted when the CAS is generated, speech, music and noise can still corrupt the signal by gaining access to the line through the near end party's transmitter. If the near end party happens to be speaking when a CAS is sent, a collision of tone signal energy and near end party speech occurs. The CAS detector connected across the tip and ring terminals observes this combined signal and must extract the CAS from within the envelope of speech. Failure to detect the CAS leads to a service failure because no calling line identification is delivered when the acknowledgment tone signal is not returned.
For applications like CIDCW, good CAS detector talkoff and talkdown performance is therefore critical. The design freedom available in previous applications to completely sacrifice talkdown performance in favor of talkoff performance can no longer be exercised. In general, the two factors that affect talkoff and talkdown performance are the architecture of the tone signal detector and the timing algorithm used to screen potential alerting signals. The architecture of the tone signal detector has the most influence on detector performance, but overall performance can be significantly enhanced by use of a "smart" timing algorithm. To illustrate how detector architecture affects performance, one of the simplest prior art architectures, shown in FIG. 1, can be considered. This detector 10 utilizes for each channel 11 and 12 a narrowband filter 13 around each signaling frequency f1 and f2. A detect condition is indicated when the power of each filtered signal coincidentally exceeds a preset threshold, as determined by threshold circuits 14, for some minimum duration of time.
The primary problem with the simple filter tone signal detector in FIG. 1 is that it is highly susceptible to talkoff. Experimentation using a digital signal processing simulation and a simple timing algorithm revealed that this circuit often responds to signal imitations produced by ordinary telephone speech. Simple filter tone signal detectors designed for single frequency tone signals are highly vulnerable to talkoff. Dual tone and multi-tone simple filter tone signal detectors are more immune to speech simulation. However, experimentation has proven that speech contains enough energy at non-harmonically related frequencies to often trigger dual and multi-tone simple filter detectors that have a low sensitivity threshold. Simple filter detectors with a high sensitivity threshold perform significantly better. They are not practical, however, for use in a network that introduces loss on the alerting tone signals. The primary advantage of the simple filter detector is its excellent talkdown performance.
To combat the talkoff problem, other prior art tone signal detectors not only measure the energy in the tone signal frequency bands, but also condition detection upon tone signal purity. The method that was used in the touch tone receivers for some of the early electronic switching systems involved comparing the power of each tone signal frequency band with the total power contained in the remaining non-signaling portion of the voiceband. The general class of these detectors became better known as tone signal detectors that employ guard. A functional block diagram of a generalized prior art tone detector with guard is shown in FIG. 2. The basic components needed to implement each single channel 21 and 22 for the guard detector 20 are a narrowband filter 23 centered around one of the signaling frequencies (f1 and f2), a bandreject filter 24 to highly attenuate the other frequencies that comprise the tone signal and a complex comparator 25 that obtains the ratio of the power of one signaling tone to the power in the rest of the voiceband and compares it against a preset threshold. The purpose of the bandreject filter 23 is to attenuate the other signaling tone frequencies so that an accurate representation of the non-tone signal energy in the voiceband can be extracted. Each complex comparator 25 produces an output signal based upon the relative power difference between the signaling tone and the energy in the non-signaling portion of the voiceband. If the ratio of the signaling tone power to the non-tone signal power in the voiceband is above a preset threshold, the incoming signal seems to be dominated by a signaling tone and the complex comparator produces a logic 1 output. Otherwise, if the ratio is below the preset threshold, the incoming signal appears to be dominated by speech energy and is likely to be a talkoff. In this case, the complex comparator produces a logic 0 output.
The signal tone and voiceband power comparison can be performed almost instantaneously in real time by analog circuitry or at periodic intervals in a sampling system. The outputs of the comparators 25 are ANDed together to form a single binary data stream that is timed by the timing algorithm. The binary data stream will contain a series of pulses that indicate the result of the comparison between the power ratio and the preset threshold as a function of time.
The guard detector of FIG. 2 permits improvements in talkoff performance because most talkoff signals can be readily identified by the presence of extraneous energy in the voiceband. The non-signaling energy in the voiceband became known as guard energy because it was used to help guard against talkoff signals. Similarly, the ratio of signaling tone power to guard signal power was called the signal-to-guard ratio. Most speech imitations would have relatively poor signal-to-guard ratios while true tone signals in the absence of interference would relatively have large signal-to-guard ratios, usually in excess of 15 dB.
Prior art guard detectors, such as one in FIG. 2, were designed for applications where talkoff was the primary concern. In fact, touch tone receivers were the primary application at the time. As previously mentioned, talkdown problems for the touch tone signaling system were circumvented by minimizing all interference when tone signals were pulsed. While the talkoff performance of guard detectors that utilize all available spectrum power is highly desirable, such detectors are a poor choice for applications like CIDCW. Because speech energy cannot be entirely controlled when the CAS is generated, there is a good probability that significant speech energy emanating from the near end party will collide with the CAS. A guard detector that looks at the entire non-signaling portion of the voiceband will almost always reject speech-corrupted tone signals and be talked down.
Another variation of the guard concept that is more convenient to implement on digital signal processors is the method of sampling the energy at a few points in the voiceband spectrum to use as guard. If the energy at any one of these points passes a preset threshold, the equivalent of the comparator output signal is set to a logic 0 as the incoming signal is thought to be a talkoff. Otherwise, if sufficient signaling tone energy is present and the energy at each of these samples is below the preset threshold, the comparator output signal is set to a logic 1.
The major advantage of this prior art design is that only a small portion of the voiceband energy is used for guard energy. This method is a further enhancement over the former guard detector because it is more tolerant to the presence of speech. If the sampled frequency points are chosen in the upper portion of the voiceband where speech power is less prominent, this detector will be able to detect more CAS signals that are corrupted by speech. By controlling the threshold, the balance between talkoff and talkdown performance can be altered. However, one drawback of this design is that sampling a few points of the frequency spectrum proves too inflexible to achieve very good talkoff and talkdown performance. The granularity in performance setting is too coarse and is much too susceptible to the position of the spectrum samples.
In virtually every tone signal detector design, talkoff performance and talkdown performance are not independent of each other. In fact, the relationship between talkoff performance and talkdown performance is typically inverse. Good talkdown performance is usually achieved at the expense of talkoff performance. The simple filter detector of FIG. 1 demonstrated this behavior. Conversely, good talkoff performance is usually achieved at the expense of talkdown performance. The guard detector of FIG. 2 exemplified this concept.
The tradeoff between talkoff and talkdown performance has an inherent logical explanation. In order to improve talkoff performance, a tone signal detector must be made more selective in what it accepts as a valid tone signal. The more selective a tone signal detector becomes, the more criteria an incoming signal must pass before it is recognized as a valid tone signal. As the detector becomes more selective and rejects more talkoffs produced by speech, it also begins to mistake valid tone signals obscured by speech as talkoffs. This in turn causes more valid tone signal to be rejected and worsens talkdown performance. For the opposite approach of improving talkdown performance, the reasoning holds that liberal detectors designed to accept signaling tones that are corrupted by speech or music are more likely to accept signal imitations produced by speech. Consequently, talkoff and talkdown performance are offsetting weights on a balance beam whose rest position is designed to favor either one or the other depending upon the application.
For every inband tone signaling application, the balance between talkoff and talkdown performance needs to be evaluated by the system engineer. A tone signal detector architecture that provides a mechanism to adjust the talkoff and talkdown balance is highly desirable. With the previous tone signal detector designs, there were two problems both of which are related to setting and controlling this balance. On some tone signal detectors, such as the simple filter detector, the talkoff and talkdown performance of the circuit was virtually fixed and unadjustable. Only slight improvements could be made by changing system variables. On other detectors such as the guard detectors, a crude and course means of adjustment was provided through the selection of the signal-to-guard ratio threshold. Adjustment via the signal-to-guard ratio provided a greater degree of control, but again, only small to moderate performance improvements could be realized. Neither of these designs could be successfully used for services like CIDCW because the correct balance of talkoff and talkdown performance could not be achieved.
An object of the present invention is to detect an alerting signal comprising multiple tones at predetermined frequencies in the presence of speech, with both improved talkoff and talkdown performance.